One of the most frustrating things about popular discussions of CRISPR is the tendency of commentators to let their imaginations run wild about potential applications. A certain degree of sensationalism is unfortunately necessary to get a lay audience interested in science, but the short-term pursuit of an audience unfortunately does some long-lasting damage to the perception of technology.

CRISPR is a powerful technology, but when it comes to altering humans, it's nowhere near as powerful as the sensationalists like to imply. CRISPR is capable of and relatively good at inserting a couple thousand base pairs of DNA into one user-defined location in the genome at a time. Considering that the average human gene is nearly ten times this length, CRISPR can really only modify or add one gene at a time. Applications as sophisticated as designer babies or humans bred to be warriors are going to require so much more editing than this that they're nothing more than scary thought experiments. Yes, the capability we're gaining to modify human reproductive tissue should give people some pause and make them a little nervous, but not because we're going to be able to engineer enslaved super-soldiers in the near future.

What CRISPR is good at is making small changes in populations of cells. I think the most promising type of applications for human health are correcting known disease-causing mutations in patient-derived stem cells. The Yamanaka method for inducing "stemness" in cells that aren't stem cells (like skin cells, for example) coupled with our ability to differentiate those cells into a variety of different types and tissues means that, in the near future, we can start engineering "replacement parts" for humans. Treating those cells with CRISPR means that we could, conceivably, remove mutations that are known to cause diseases and supply patients with functional cells and tissues. This is absolutely something worth getting excited about!

The potentially appalling applications of CRISPR aren't in humans -- they're in non-animal species that have fast generation times and can have large populations maintained in labs -- like insects. We're currently capable of engineering self-propagating edits that alter the behavior of individuals; these individuals can then be released into the wild and spread their edits throughout entire populations. This is essentially ecological engineering, and it can be accomplished by small groups with initiative. There's a lot of potential for abuse there, but I don't think it's quite enough to make CRISPR an appalling prospect.